Electrosurgery involves the application of radio-frequency electrical energy to tissue to produce a surgical operation. Electrosurgery is generally performed with a generator that converts electrical energy from a power source to a predetermined radio-frequency waveform that is delivered to the tissue through an active electrode and a return path.
There are essentially four main surgical operations that are electrically performed on tissue, depending on the radio-frequency waveform output by the generator. These operations are typically described as desiccation, fulguration, cutting and cutting with hemostasis.
For a desiccation operation, the generator outputs a radio-frequency waveform that heats the tissue, by electrical resistance heating due to current flowing through the tissue, sufficient to produce an area of necrosis.
For a fulguration operation, the generator typically outputs a burst waveform which has a high peak voltage but a low duty cycle. Due to the low duty cycle of the fulgurating waveform, the power per unit time applied to the tissue is low enough so that explosive vaporization of cell moisture is minimized. The burst waveform forms a radio-frequency spark or arc between the active electrode and the tissue, thereby delivering power over the area of the spark or arc tissue contact and providing coagulation of the tissue in the immediate vicinity of the spark or arc.
Other operations can be performed with still different waveforms output by an electrosurgical generator. Cutting occurs when sufficient power per unit time is delivered to the tissue to vaporize cell moisture. Cutting is typically performed with a repetitive voltage waveform, such as a sinusoid, which produces a cut with very little necrosis and little hemostasis.
It is also possible to achieve a combination of the above operations by varying the electrical waveform produced by the generator. In particular, a combination of cutting and desiccation (called cutting with hemostasis or blend) can be produced by periodically interrupting the continuous sinusoidal voltage normally used to produce an electrosurgical cut.
Known electrical generators which are capable of producing one or more of the above-described surgical operations are generally designed as in FIG. 1. The AC power mains 200 provide AC electrical power to AC/DC converter 202, which provides unregulated DC power to the DC regulator 206. Under the control of clinician 208, control and timing circuitry 210 causes the DC regulator 206 to produce DC power of a specified value to the tuned RF amplifier 212. The control and timing circuitry 210 also produces RF signals for amplification by the tuned RF amplifier 212. This results in RF power signals being delivered to the patient 214.
Known electrosurgical generators are subject to one or more limitations. For example, some generators are limited in the degree to which they can generate more than one individual waveform without producing an admixture of inappropriate effects, thus they are limited in the number of electrical waveforms that are appropriate for surgical operations.
Another limitation is that known generators emit a substantial amount of electromagnetic interference to the environment. Electromagnetic interference poses a serious risk in operating rooms where it can cause malfunction or failure of electronic equipment. A primary source of the electromagnetic interference is the substantial pulsating currents which are created in electrosurgical generators circuits.
There are primarily two sources of electromagnetic interference (EMI) in known generators. Such EMI consists of conducted EMI, nearfield EMI and radiated EMI. A primary source of the conducted EMI, which is sent back into the AC power lines and carried to equipment at distant locations in the hospital and beyond, is produced by the substantially pulsating currents which are created in the DC regulator 206. A primary source of the nearfield and radiated EMI is the harmonic content of the tuned amplifier output. The harmonic components couple much better to the environment, and are radiated away more effectively. As will be shown, a key aspect of this invention is the simultaneous reduction of conducted, nearfield and radiated EMI.
Another limitation of known electrosurgical generators is their relatively low efficiency in converting and amplifying electrical power from the power source to the tissue, resulting in the dissipation of electrical energy as heat. Heat dissipation by an electrosurgical unit (ESU) within an operating room is objectionable due to the generation of convective air currents and the associated circulation of airborne pathogens. The additional heat dissipation requirement increases the weight and volume of the ESU. Furthermore, the reliability of the electrosurgical unit typically decreases as the heat dissipation increases.
Low efficiency in ESU's are caused by a number of effects:
(1) Topology selected, which determines intrinsic efficiency (maximum achievable efficiency under optimum conditions); PA1 (2) Loading, which determines extrinsic efficiency (efficiency achieved with given topology into a given load); PA1 (3) Component selection, which determines realized efficiency (efficiency with a given topology, load and selection of components).
Ideally, a topology is selected which maximizes the extrinsic and realized efficiency over a wide range of conditions. In known ESU's, in order to achieve cutting with a minimum of hemostasis, AC ripple voltage present on the DC regulator output should be minimized. At the same time, the conducted EMI should be reduced as much as possible. To do this, large size capacitors are sometimes added to the AC/DC convertor 202, in FIG. 1, in an attempt to smooth the current pulses, reducing conducted EMI, while at the same time large capacitors are added to the output of the DC regulator 206 to reduce output ripple and hence reduce hemostasis. These capacitors filter the current by passing the ripple component to ground through the ESR of the capacitor, thereby wasting power. This loss and bulk would be greatly reduced if less AC ripple were generated, and hence less power wasted.
Control devices, such as transistors, are often used in both the DC regulator 206 and the RF amplifier 212 circuits to synthesize and regulate the electrical waveform applied to tissue. These control devices may be used in a variety of ways. A very common method in prior art has been to use the control devices as variable impedance current sources which results in the simultaneous application of voltage and current across the transistor and thereby a dissipation of power within the transistor. Control devices are also used as alternating low impedance (i.e., closed) and high impedance (i.e., open) switches. In prior art, some generator circuits dissipate a substantial amount of power in such switches due to transitioning the switches to low impedance while a voltage exists across the switch and thereby dissipating power due to the simultaneous presence of voltage and current in the switch. Some topologies of generator circuits which contain transistors often cannot tie the biasing of the transistors to a common reference node, thereby requiring relatively complicated level shifting circuitry.
Some known electrosurgical generators' topologies convert the input voltage to an output voltage through a process that includes storing input energy inductively in the form of a DC magnetic field during one interval and releasing the energy as an oscillating voltage across a load during a subsequent interval. This process of storage and release of energy results in a waveform in the form of a damped sinusoid which has a significant amplitude remaining at the time of the next storage cycle. For some output waveforms, such as pulsed energy waveforms, energy not sent to the load by the end of the pulse remains in the generator where it is dissipated as heat, decreasing the generator's efficiency.
Consequently, there is a need for a generator that addresses such limitations of known electrosurgical generators.